Expand WTF::StringView's API to be more like StringPiece.

third_party/WebKit/Source/wtf/wtf.gypi

 # For GN compatibility, this file can not have any conditions.
 {
     'variables': {
         'wtf_files': [
             'ASCIICType.cpp',
             'ASCIICType.h',
             'AddressSanitizer.h',
             'Alignment.h',
             'Allocator.h',
             'Assertions.cpp',
@@ skipping 137 matching lines @@
             'text/StringHash.h',
             'text/StringImpl.cpp',
             'text/StringImpl.h',
             'text/StringImplCF.cpp',
             'text/StringImplMac.mm',
             'text/StringMac.mm',
             'text/StringOperators.h',
             'text/StringStatics.cpp',
             'text/StringUTF8Adaptor.h',
             'text/StringView.h',
+            'text/StringView.cpp',
             'text/TextCodec.h',
             'text/TextCodec.cpp',
             'text/TextCodecASCIIFastPath.h',
             'text/TextCodecICU.cpp',
             'text/TextCodecLatin1.cpp',
             'text/TextCodecReplacement.cpp',
             'text/TextCodecReplacement.h',
             'text/TextCodecUTF16.cpp',
             'text/TextCodecUTF8.cpp',
             'text/TextCodecUTF8.h',
@@ skipping 57 matching lines @@
             'text/StringImplTest.cpp',
             'text/StringOperatorsTest.cpp',
             'text/TextCodecTest.cpp',
             'text/TextCodecReplacementTest.cpp',
             'text/TextCodecUTF8Test.cpp',
             'text/WTFStringTest.cpp',
             'typed_arrays/ArrayBufferBuilderTest.cpp',
         ],
     },
 }